(1) Field of the Invention
This invention is concerned with nontacky shaped objects of normally tacky elastomers, and a process for preparing them.
(2) Description of the Prior Art
It is a well known process to emulsify polymerizable monomers in water and then convert them to polymers in dispersed form. The resulting dispersions are called latices. Natural rubber also occurs as a latex. Some latices are highly useful as formed, for example, basic formulations for latex paints. For other uses, it is necessary to coagulate the polymer and separate it from the latex. The most important commercial form for these polymers is as washed and dried, nontacky shaped objects. These shaped objects should be washed until they contain less than about 3% by weight of water soluble material, and dried until they contain less than about 1% by weight of volatile material.
If the coagulated polymer is inherently nontacky, the polymer is readily isolated from the aqueous phase, washed and dried to form a free-flowing powder which is readily handled in further processing. If, however, the polymer is inherently tacky, coagulating of the latex yields the polymer in an agglomerated mass which is difficult to wash free of impurities and also difficult to dissolve, to blend with other materials, and to feed in compounding, curing and molding operations.
In the field of natural and synthetic rubber, much attention has been given to the problem of agglomeration of normally tacky polymers before curing. Some operable solutions have been found.
In U.S. Pat. No. 2,879,173, Yacoe shows a process for coagulating a polychloroprene latex by suspending drops in a volatile, water-immiscible, organic liquid in which the polychloroprene latex is insoluble while maintaining the liquid below about -20.degree. C. until the drops are completely frozen and coagulated. The resulting frozen pellets are separated and coated, while still frozen, with from 5% to 20% by weight of an inert powder, such as talc, so that they will not stick together when thawed and dried. However, the use of this process has been limited by the cost of organic liquid recovery, the energy cost of the refrigeration, and the undesirable high loading of inert material. The need for a more practical and economical method of preparing nontacky, washed and dried shaped objects of normally tacky elastomers remains.
In U.S. Pat. No. 3,053,824, Heinz shows coagulation of a phosphate or borate buffered elastomer latex by running it into an aqueous aluminum or titanium salt solution containing also an alkali metal or alkaline earth metal salt. The patent states that the resulting coagulated particles do not stick together before being washed to remove the coagulating salts. There is no statement as to the tack of the particles after washing. Comparative Example 1(d) shows that, when Na.sub.4 P.sub.2 O.sub.7 is left out of the latex, the precipitate is very sticky and after a time forms lumps. Accordingly, the nonsticky character of the product is associated with the presence of Na.sub.4 P.sub.2 O.sub.7 in the latex.
In U.S. Pat. No. 3,846,365, Berg et al. show a powdery, filled polymer prepared by emulsifying together a dilute solution of an elastomer in a volatile organic solvent and a solid, finely-divided filler for the elastomer, passing the emulsion into an aqueous alkali silicate solution at a pH preferably of 7 to 12, and evaporating the organic solvent. A finely-divided, tack-free coprecipitate of the latex and silicic acid residue is formed. It is stated that condensation of the silicate anions should be avoided.
Neoprene is commercially prepared at the present time by destabilizing an aqueous chloroprene latex bath by adding acetic acid close to the coagulation point of the latex, passing the latex to a freezing roller at -20.degree. C. to form a film, washing the film to remove primarily sodium acetate, and drying the film at 120.degree. C. This film, which is highly tacky, is then coated with talcum, chopped and refrigerated. Since this process involves refrigeration and coating with talcum, a less expensive procedure which requires smaller amounts of inert material would be desirable.